Spiral Matrix II

Given a positive integer n, generate an n x n matrix filled with elements from 1 to n2 in spiral order.

Example 1:

Input: n = 3
Output: [[1,2,3],[8,9,4],[7,6,5]]

Example 2:

Input: n = 1
Output: [[1]]

 

Constraints:

• 1 <= n <= 20

Solution:

class Solution {

    public int[][] generateMatrix(int n) {

        int[][] matrix = new int[n][n];

        int top = 0, bottom = n - 1, left = 0, right = n - 1;

        int val = 1;

        while (val <= n * n) {

            for (int i = left; i <= right; i++) 

                matrix[top][i] = val++; 

            top++;

            for (int i = top; i <= bottom; i++)

                matrix[i][right] = val++;

            right--;

            for (int i = right; i >= left; i--)

                matrix[bottom][i] = val++; 

            bottom--;

            for (int i = bottom; i >= top; i--)

                matrix[i][left] = val++; 

            left++;

        }

        return matrix;

    }

}

Diagram in the problem statement clearly explains about matrix flow. Now, we are going to code accordingly by following the above diagram.

1) First for-loop is used to fill top row.

2) Second for-loop is used to fill right column.

3) Third for-loop is used to fill bottom row.

4) Fourth for-loop is used to fill left column.

5) Repeat above steps till val <= n*n. For any n, it will be executed and rotate spirally and fill entire matrix.